Beam Profile Reflectometry (BPR) and Beam Profile Ellipsometry (BPE), described in U.S. Pat. Nos. 4,999,014 and 5,042,951 respectively, are established methods for measuring the thicknesses of thin films and coatings deposited upon flat substrates. Each of these techniques makes use of data contained in the cross-sectional profile of a laser beam which has been reflected from the sample under test. As described in each of these patents, each technique relies on the sample under test being (a) perfectly flat and (b) aligned so that an incident probe beam is focused substantially normal to the surface of the sample.
In practice, known implementations of BPR rely on a very high degree of accuracy in achieving this alignment, since only when the alignment is perfect are the beam profiles obtained symmetrical, and this in turn is an assumption made in the subsequent analysis. To this end, the present inventor has devised a sophisticated five-axis stage arrangement (described in WO2008/119982) which, when used in combination with optical feedback from a BPR system, can be used to align a sample (including samples with complex shapes) to an arbitrarily high degree of accuracy.
However, the requirement for this alignment (whether achieved statically for flat samples by very accurate construction of supporting hardware, or dynamically for curved surfaces using a stage such as that referred to above) introduces significant cost and complexity into a BPR or BPE system.
It is therefore an aim of the present invention to provide an apparatus and method that addresses at least some of the afore-mentioned problems.